


Test Subject

by 5Lifelines, Cherry4242red



Series: TsukaSen Is My Religion [5]
Category: Dr. STONE (Anime), Dr. STONE (Manga)
Genre: Alternate Universe - Modern Setting, Bathing/Washing, Bold Senkuu, Dead Dove: Do Not Eat, Discord: The Kingdom of Shipping (Dr. STONE), Dubious Consent, Heavy Petting, M/M, Modern Era, Senkuu and Tsukasa are 10&12 or 11&13, Senkuu and Tsukasa are friends, Sleepovers, Teasing, TheKingdomofShipping, Whatever helps you go to sleep at night, intended for mature audiences only, read the tags and avoid content that triggers you im begging :|, this content is not for children - seriously if you're under 18 gtfo
Language: English
Status: In-Progress
Published: 2019-11-24
Updated: 2019-11-24
Packaged: 2021-02-26 03:48:24
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 3,537
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21547108
Author URL: https://archiveofourown.org/users/5Lifelines/pseuds/5Lifelines, https://archiveofourown.org/users/Cherry4242red/pseuds/Cherry4242red
Summary: Tsukasa is having his first sleepover at Senkuu’s house, but unbeknownst to him, he’s apart of one of Senkuu’s more… salacious experiments.
Relationships: Ishigami Senkuu/Shishiou Tsukasa
Series: TsukaSen Is My Religion [5]
Series URL: https://archiveofourown.org/series/1581793
Comments: 11
Kudos: 91





	Test Subject

**Author's Note:**

> This originally started as an RP, but we figured we might as well share it with you all.
> 
> 5Lifelines is Tsukasa and Cherry4242red is Senkuu.
> 
> This fic contains underaged characters, if that puts you off, I'd suggest closing out this fic now.  
> Senkuu is 10/11 here and Tsukasa is 12/13. Imagine this is during the first "fake" flashback that Tsukasa conjures up in his mind, if he and Senkuu were friends back in modern day.

The door closes behind him after Senkuu’s father finishes his small talk with the young boy, leaving him and his friend alone in his bedroom. Tsukasa is a tad nervous, since this would be his very first sleep over and the fact that it was just going to be him and Senkuu was a little more than overwhelming. The two of them usually were accompanied by Taiju, who would help ease his anxieties. Not because he and Senkuu didn’t get along, but because it was a good way to distract him from his thoughts toward the little scientist. He wasn’t often afraid, but when it came to Senkuu his heart would beat a mile a minute and his palms felt consistently sweaty. He whipped his hands on his shorts and proceeded to greet the other with the only words he could muster at the time.

“Hey, Senkuu!”

Senkuu has been looking forward for this sleepover all week, it wasn't common for him and Tsukasa to spend time alone ever since he'd known the other boy.

  
But that wasn't the real reason Senkuu was looking forward for this, no, it was something way more interesting.

"Hey Tsukasa!" Senkuu smiled in acknowledgement at the other kid, as he made his way to sit on the bed.

You see, recently he started noticing...unusual things about Tsukasa whenever Senkuu was too close to him. Or whenever he wore shorts and sleeveless tops. Or when they would go swimming. And tonight he decided to put his theory into testing.

Tsukasa takes in a deep breath, he needed to calm down.. There wasn’t anything to be worried about, it was just him hanging out with his friend. Sure, it was the friend he couldn’t stop thinking about, dreaming about, …fantasizing about– He had to repress it, he wasn’t alone in his room right now, Senkuu was there right in front of him and he needed to be normal, just for one night. Senkuu went to his bed and he decided to follow suit and sit next to him. He started to fidget a bit, his hands gripping at the bedsheets and his feet tapping the floor unrhythmically.

He wanted to say something, anything, but he lost the will to speak. He repeated over and over in his mind that now wasn’t the time to act like a teenage girl with a crush, it was time to just be normal. It was unsuccessful, clearly as he sat next to him in silence.

Senkuu's eyes focused on the nervous figure next to him, the way Tsukasa avoided eye contact, the way he kept his hands busy as if to distract them.

  
The corner of his lips turned up forming a subtle smirk.

"So...," pausing a little to make sure the other has enough time to turn his attention back to Senkuu. "Wanna play some video games? " the seemingly innocent question. Had a something hidden into it.

"We can play Mario together, " he suggested. Slightly inching his upper body towards Tsukasa. Taking in the way the other's pupils widened a bit at his action.

The space between them becoming smaller was unexpected, but not unwanted – thoughts of being close to Senkuu riddled his mind daily, after all. Yet, he couldn’t help but scoot away from him instinctively. Hopefully he wasn’t coming off as rude, but it wasn’t often that anyone would get close to him like that with the exclusion of adults with – for lack of a better word – ill intensions. He was more than capable of defending himself from those kinds of folks though and he wouldn’t exactly be opposed to Senkuu making what some might call questionable advances at him. He could only dream of such a thing.

“Video games? We could, I’m not very good at them, but if that’s what you’d like to do, I’d be glad to play with you, Senkuu.”

It was true, video games weren't a big part of his life. He often saw it as something those with a better home life had the pleasure of taking part in. Though, if this is what his Senkuu wanted to do, he was more than happy to take part, even if he’d be terrible at it. In his mind, his sole purpose was to keep his friend happy and to protect him.

"Oh, don't worry. It doesn't matter if you're good at it or not, it's going to be fun " technically it wasn't a lie, After all. Senkuu just wanted to catch Tsukasa off guard.

His small legs were barely touching the floor from where he sat, although used to it. He managed to get down, heading towards the TV with the console attached to it. He pressed the button and soon after the TV screen lit up.

Still standing from where he was, he pointed at the empty sitting space in front of the TV, gesturing to Tsukasa to join him.

“Alright!”

Without any hesitation, he was sure to do what was expected of him, sitting where the other boy requested of him. He got comfortable in his seat and grabbed hold of the controller, playing around with the buttons and knobs to get a hang of the small device.

“What kind of game is this?”

He knew of the Mario character, but not really about the context of the different games he was featured in. Whatever the context, so long as he was able to spend time with his friend, he was sure to have a good time.

Senkuu's smirk grew. As he watched the other boy fall right into his trap. Just like a rabbit tricking a lion. "Oh, you know, jump around, punch the bad guys, save the princess" he picked his own controller.

"How about I teach you how it works," his tone not sounding so innocent, as what followed proved that to be true.

Senkuu made sure Tsukasa would take a seat before him, as he slowly turned to sit on the taller boys lap, making himself comfortable as his hands grabbed Tsukasa's. "You use this button for jumping, " he gestured as he made sure his fingers would touch the other's. "This one for hitting, and this for moving.” Turning his head around, hoping to see the fruits of his experiment earlier than expected.

This. Was. Bad.

His pre-teen brain was short circuiting, his muscles were tensing, and he was hoping his body wouldn’t have any perverse reactions to having Senkuu’s ass pressed against his crotch. The younger boy’s hands were so soft and warm atop his own, as he guided his hands to the various buttons on the controller. This was way more stimulation than he was ready for all of a sudden. Weren’t they just supposed to be playing video games? What was Senkuu doing in his lap like this – touching him in this way? He needed to clear his mind, he was making this out to be something that it’s not. He was so hopeless, seeing something that wasn’t there.

When the little scientist turned around to face him, he was so close – if they were any closer their lips would be pressed together. Tsukasa breathed heavily out of his nose as he realized he’d been holding his breath this entire time.

“Th-thanks for your help, but… is this just a one player game? Don’t you – don’t you wanna play something we both can play together?”

Senkuu felt so happy, like some devilish scientist seeing the body he just assembled starting to move, Tsukasa's reactions were lining up with his suspicions so far!

"No-no, that was the _old_ version! The new one has a two player's mode," his hand reaching to pick the controller he left neglected on the ground, bending on the other's lap in the process, as he made a show of adjusting himself there again when he came back up.

Deciding to push things further, he pressed his entire back to the other's chest, so the only way to look at each other was for their lips to be inches apart, now he just has to keep track of Tsukasa's pupils size and his hands movement, pressing his back further to be able to feel the other's heartbeat.

"Now let's play," was the last setback that left his lips before he pressed start on the controller. A taller green version of Mario entering the screen.

Tsukasa let out a soft moan as Senkuu pressed into his crotch once more. With each movement from the younger boy, he found himself heating up more and more. As Senkuu settled into his lap and pressed his back to his chest, the browned haired adolescent found himself getting hard. He closed his eyes tightly as he tried to think of anything to reverse the stiffening in his pants but to no avail. All he could do was hope that Senkuu wouldn’t notice. If he was lucky, Senkuu might be just young enough to not know what was going on with him. He was a couple years older than him after all.

Tsukasa was so stuck in his thoughts, he honestly didn’t hear a word Senkuu said. He merely murmured out a “mhm,” and proceeded to mindlessly press the controller buttons. He found himself dying repeatedly.

Senkuu's brain was in paradise. His last experiments ended up failing, so this one going extraordinarily well was like a breath of fresh air. He pretend to not hear the moans the other would let out whenever Senkuu moved. Or how he was intentionally moving and huffing every time Tsukasa's character died.

He also made sure to pretend to not notice the hard thing poking his bottom from underneath...now thinking about it. He read books about the male genitalia...but he never got a chance to actually see one in real life besides his own of course.  
"Hey Tsukasa, what's that hard thing in your pocket" deciding to push things further. He needed proof that it was him causing this reaction out of Tsukasa and not simply puberty and random hormonal boners.

His eyes started to widen a bit and his face started to become a more crimson color as he took in Senkuu’s question. Of course he wasn’t going to tell him, “ _Oh, that’s my dick, it’s hard because you keep rubbing your ass against it and turning me on._ ” Yeah, no that wasn’t about to happen. Since, Senkuu was asking him a question like that, this must mean he was right, Senkuu was just young enough to still not have any idea what was going on with his body. Perhaps, he could use this to his own advantage and play dumb. If he was lucky, the boy would move on and continue to focus on his terrible gameplay and not his prominent erection.

“Hard thing? I’m not sure what you’re talking about, Senkuu.”

He let out a nervous chuckle and prayed like hell that this conversation would come to an end shortly.

Senkuu's expression changed into the same one when Byakuya told him the moon had a crush on him. Oh, so he wants to play dumb now? Quickly changing it back to the innocent one. He dropped the controller on the ground. As both his hands went to grasp the hardness under his ass.

"I'm talking about this. What's this, Tsukasa?" His hands squeezing the bulge of the other's crotch a bit. As his eyes innocently looked into the blushing figure behind him. Taking how the other's pupils increased in size and his breathing became heavier.

“Ah – Ahh~”

His moans came out fully, pathetic and needy. He was caught of guard and there was no chance of him holding back. He only hoped that Byakuya was hard of hearing or asleep in the other room. Unlike his friend here, there was no way for him to lie to an adult about his lil “problem.”

The small delicate hands of the one he’s been fantasizing about since their friendship started - were actually cupping at his crotch. If he thought he wasn’t short circuiting before, he definitely was now.

“S-Senkuu, what are you-?”

Curious, he was just curious, right? No need to lose himself here.

"What do you mean, Tsukasa?" Senkuu’s hand continued to grasp and pull at the other's clothed erection. Maybe if he gets Tsukasa to soil his pants, then he will get to see what's underneath and compare it to his existing data.

"I'm just trying to figure out what this thing is. You know, it has been poking me since I sat down. It's really annoying you know," he said as he laid his head back on Tsukasa shoulder.

"Why are you letting these sounds out Tsukasa. Are you in pain or something? Do I have to call Byakuya," he whispered in the other's ear. His tone holding a bit of amusement to it. That would definitely go unnoticed by the person next to him.

“I-I’m sorry, I don’t mean to annoy you.”

His voice was getting shakier. Senkuu doesn’t know just how much this was affecting him – what this was doing to him. His cock was twitching in his shorts and he desperately wanted to rescue it from this hellish cloth prison.

“I’m fine, there’s no – no need to call for him.”

He’d give anything to feel the direct contact, these clothes really were a bother, but what if Senkuu realized what was going on with him? Maybe not now, but later and realizing he was a disgusting person, lusting after his friend, his male friend no less. He wouldn’t want to continue being friends with him. The last thing he wanted was to lose his best friend, because of his stupid hormones.

Alright, now for the final step. To make sure Tsukasa was getting hard because of him and not because he was imagining some hot busty lady in his mind.

He pulled one of Tsukasa's hand with his own. As he slowly dragged it to his shorts. Where Tsukasa would feel something small poking out. He was gonna force Tsukasa to face that he is with another guy right now

"Tsukasa… it's very weird. It's happening to me too," he pretend to be oblivious. As he watched closely. His other hand still grasping Tsukasa's groin. Waiting to see if his boner will fade when Tsukasa touches him too.

"Mmmph...”

He bit his lip, if he bit any harder he’d be sure to draw blood. He was perspiring now and his breathing was becoming more shallow and erratic.

_Shit. Is this really happening?_

Tsukasa’s hand trembled slightly as he went over in his mind how he should react. Swat his hand away? That could potentially offend or upset Senkuu. So, that was a no. He could continue to do nothing in hopes that Senkuu will willingly move his hand away due to a lack of response. Though, he seems curious and maybe even wanting an explanation, so he can’t just sit there and do nothing…

His hand seemed to gain a life of it’s own due to his indecisiveness, pressing into Senkuu’s clothed erection. He was smaller than himself, just as he imagined – nice and cute – now if only he could see it with his own eyes. He was sure he was uncut too, the curiosity was killing him. There’s no Senkuu wouldn’t think he was a disgusting person, taking advantage of his naivety. He was starting to leak now, the pre-cum seeping through his shorts. There was no ignoring this now.

Senkuu unexpectedly let out a gasp when Tsukasa hand pressed into him, he didn't imagine that Tsukasa would participate… wait maybe there was more to Tsukasa than just being gay...maybe Tsukasa actually liked Senkuu in that way.

The new theory blossomed in Senkuu's brain, he was getting more questions than answers out of this experiment, and he wasn't satisfied with that, he **will** get his answers.

And just like that, he felt wetness under his other hand that gripped Tsukasa's groin...oh... _oh_ so it turned out his theory was right. Tsukasa is in fact gay. Happy with his new discovery. He removed his hand from the other’s groin. Bringing it to his lips as to lick the remains of the sticky clear fluid that was stuck to it. Now that his experiment was done he could focus on other things. As he attempted to lift himself up from Tsukasa lap.

Tsukasa was sad to see and feel the removal of the other’s hand, but the sight that proceeded shortly after had his eyes fully dilated. He whimpered slightly as he felt himself leaking more through his clothes. If only Senkuu would use his tongue on him instead of indirectly.

With Senkuu removing himself from his lap, he began to feel a bit lighter but longed to have the weight placed on him again. He wasn’t sure what to expect next, but if it continued in this direction he was afraid of what he’d do to his innocent friend. If only he knew the result of his actions….

Now. Senkuu was very happy about his now proved true theory, and he wanted to keep this streak going and answer as many theories as he can with Tsukasa nearby. Although that might be a bit hard to do with Byakuya in hearing range.

His next theory was. How far can he push a human before their instincts takes over. Of course he knew it depended from one to another. But he found Tsukasa to be a good base start for his research. He needed to give Tsukasa just enough simulation that he will want more, but not too much that he will take it for granted.

Tsukasa adjusted himself in the seat to make it so that Senkuu could not see his hard on. He was genuinely starting to worry if he scared the other away, though it didn’t quite explain the fact that he licked up his pre-cum and didn’t give him a snide remark about it being gross or anything. He was really at a loss, but he surely didn’t want to put his friendship in jeopardy.

“Do you no longer want to play with me, Senkuu?”

The didn’t want to hear Senkuu respond with a yes, he didn’t want to have to leave, he wanted to spend more time with him. Why couldn’t he just be a normal boy instead of being like this?

Senkuu decided that the first step to achieving his goal. Was to drop some of the innocent act. After all. He doubts Tsukasa would suspect him due to some light teasing.

"I do Tsukasa, but dinner will be ready soon and taking a bath before it will give us more time to play after.”

He started counting. Experiment two has begun.

"After all. The hard thing I was sitting on, managed to soak a bit of my shorts," Senkuu walked towards the drawers as he got some towels for them both.

"We don't want Byakuya finding our about our playing. Now do we Tsukasa?”

He gulped, the last thing that he’d want is for Senkuu’s father to see them like this. With him being the eldest one, he’d surely get some reprimanding. He should be the one to have control of the situation between the two of them. His father was a nice guy, but he’d hate to see his more serious side when it comes to protecting his some from is perverted friend who has his pre-ejaculate on his son’s ass.

“You’re right we should, deal with this,” he said hesitantly. He realized that Senkuu was suggesting they take a bath and dispose of their soiled clothing, but was he expecting them to bathe separately or together?

“Who’s going to go first?”

Maybe if he was a bit younger, it wouldn’t be too weird for them to bathe together? He didn’t wanna be the one to suggest it. He wasn’t sure he could handle being naked with Senkuu like that. He was having enough trouble as is.

Senkuu just raised an eyebrow at the other. "And waste perfectly good hot water, Tsukasa? I expected better than that from you," Senkuu's remarks were getting bolder.

Making his way to the door as he gave Tsukasa some time to get his erection under control so they can roam freely outside the room without much suspicion.

"Plus. Maybe we'll finally know what's the source of that sticky stuff that came out, it was a little bitter" Senkuu made a show of licking his lips as if to remember the taste while staring the other in the eyes.

_I’d be more than happy to show you were it came from, so long as you use your tongue on me. Ugh, no - stop thinking about it..._

Okay, palms were one-hundred percent getting sweaty again.

“You – You’re right, of course it makes more sense to go in together,” he tried is best to calm himself down so his erection would begin to subside. He nodded at Senkuu to express that he was ready to leave the confinements of his bedroom.

He was beyond fucked. This sleepover was becoming battlefield between his common sense and raging hormones and one side was surely at a disadvantage.

**Author's Note:**

> Hope you enjoyed the story so far, next chapter should be out soon :3
> 
> If you are interested in joining a Dr. Stone server, feel free to join us on Discord! :D 
> 
> <https://discord.gg/fakzTPp> (ALL ships welcome!)


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